A device of the above-mentioned kind is previously known from U.S. Pat. No. 4,501,950. A disadvantage of the known device is that it requires a large space in the lateral direction, which means that it is not suitable for carrying out welding operations in narrow spaces, for example inside car bodies.
If one should attempt to reduce the size of a device of the above-mentioned kind by placing the sensor nearer the welding torch, there will arise problems as to how to protect the sensor against the radiation heat of the welding arc and how to protect the optics from being clogged by the welding smoke. In prior art seam trackers these problems have not found any satisfactory solution.